In wireless data communication systems, information bits are modulated before transmission. Traditional modulation methods include amplitude modulation (AM), frequency modulation (FM) and phase keying modulation (PSK).
In these modulation schemes, a sequence of information bits is mapped to a signal with a unique amplitude, phase and/or frequency. Accordingly, the receiver will perform demodulation to convert received signals back to the information bits.
An example transmitter 10 is illustrated in FIG. 1. A channel encoder 12 provides information bits 14 to a serial-to-parallel converter 16. The output of the serial-to-parallel converter 16 provides 5 channel bits 18(1)-18(5) to a signal mapper 20. The signal mapper 20 maps the 5 channel bits 18(1)-18(5) to a channel symbol 22. Based upon the modulation performed by the signal mapper 20, the modulated channel symbol 22 is then transmitted over the air by a transmit antenna 24.
Each modulation scheme has a bit error rate, and this bit error rate is asymptotically determined by the minimum Euclidean distance between two distinct modulated symbols. To achieve high data rates in evolving wireless standards, high-order modulation schemes such as 16 QAM and 64 QAM are used.
However, these high-order modulation schemes require a higher signal-to-noise (SNR) ratio to achieve certain bit error rates (BER). Modulation schemes that require high SNR to achieve certain bit error rates are adversely affected in multipath fading environments, thus causing the wireless links to be unreliable.